This invention relates to an apparatus for separating the background of random counts from the total of counts obtained in the gathering of data to produce a net pulse rate proportional to the activity of gamma rays in a desired energy range.
In many fields that require digital signal conditioning it is desirable to subtract a statistically variable background signal from the detected gross gamma signal. It is also frequently desirable to monitor the difference over a wide range of count rates. That continuous difference is referred to herein as a dynamic difference, since it comprises a continuing subtraction in real time. Such a dynamic difference is particularly desirable in gamma spectrometry where the resultant difference signal is used for continuous recording or process control, or both, in real time. Such circuits have been constructed in the past using analog electronic circuits. One channel is normally available that contains the combined set of pulses of the gamma rays being measured and the undesired background pulses. A second channel is set up to respond only to a signal proportional to the undesired background pulses, which signal in general has the same statistical properties as the background in the signal channel but is not necessarily coincident therewith in time so that direct subtraction is not effective. Past practice has been to convert the pulse output of the signal channel to an analog d-c signal and to convert the background pulses to another d-c signal. The two are subtracted in a d-c subtraction circuit to obtain a difference signal. Such a circuit has two major disadvantages. First, the dynamic range is relatively small, being about 2 decades without a manual change range setting of the count-rate circuits. Even with the use of log count rate circuits the loss in accuracy becomes significant over a large dynamic range. Second, any d-c drift of the analog circuits appears as a measurement error since it cannot be separated from the d-c analog signals.
It is an object of the present invention to provide a better circuit for obtaining the difference between a signal including noise and the noise signal over a large dynamic range, of the order 6-8 decades.
It is a further object of the present invention to provide a digital circuit for subtracting a noise background from a signal including noise of the same statistical parameters.
It is a further object of the present invention to provide a dynamic pulse difference circuit having a range of useful operation on the order of 6-8 decades using simple circuitry.
It is a further object of the present invention to provide a dynamic pulse difference circuit that is not sensitive to errors caused by d-c drift.
Other objects will become apparent in the course of a detailed description of the invention.